The present invention is directed to an improved etchant and a method of etching high aspect ratio openings in silicon oxide layers. More particularly, the present invention is directed to an improved method of etching high aspect ratio openings in silicon oxide layers using a fluorocarbon etchant mixture.
As the density of semiconductor devices on a silicon-containing substrate becomes higher and the size of the devices becomes smaller, etched openings made in various layers must have a smaller diameter, and thus openings have a higher aspect ratio (ratio of depth to diameter). This raises several serious problems. The selectivity, or difference in etch rate, between a patterning layer, such as a mask or photoresist layer, and the layer to be etched must be sufficient so that the resist is not etched away before the opening is made through the layer to the substrate. The selectivity between the layer to be etched and the substrate must also be such that the etch will stop when the substrate material is reached. Further, in order that the opening size is about the same at the top of the dielectric layer and at the bottom of the dielectric layer, straight walled, vertical openings must be made.
In recent years, the integration level of LSI devices has been increased, and two conductive layers separated by a dielectric layer, generally silicon oxide, are connected by means of vias made in the dielectric layer which are filled in with metal contacts. The dielectric layer must be planarized, as by chemical mechanical polishing, prior to depositing a second conductive layer thereover, and thus the dielectric layer must be fairly thick. When high aspect ratio, straight-walled openings are to be made in silicon oxide over a silicon-containing substrate using a fluorocarbon etchant, another problem arises; the etch stops after a certain depth has been reached, and prior to reaching the substrate. This is believed to be due to the deposit of unreactive, fluorine-containing polymers on the bottom of the opening that greatly slows or even stops the etch. The addition of oxygen to the etch gas has been tried to burn off the polymer deposit by oxidation or ashing, but has not been successful because the addition of oxygen also increases the etch rate of the mask layer so that the mask layer is etched away before opening have been made through the silicon oxide layer. Further, oxygen etches the mask layer isotropically, destroying the fine mask patterning. Thus the mask layer must be thickened, but this in turn makes it more difficult to etch straight walled, vertical openings because the aspect ratio is even higher than before. Current requirements for the mask layer are that it be as thin as possible to reduce the aspect ratio of the etched opening. Thus any corrosion or isotropic etching of the mask layer creates serious problems.
Still further, the etched opening should have straight, vertical sidewalls with a minimum of difference between the diameter at the top of the etched opening and the diameter at the bottom of the etched opening.
Thus a method of etching high aspect ratio, small diameter openings in silicon oxide layers has been sought that will etch anisotropically, eliminate the etch stop phenomenon, provide high selectivity between a mask or photoresist layer and the silicon oxide, and provide high selectivity between the silicon oxide and a silicon-containing substrate.
We have found an etchant that will plasma etch very small diameter, high aspect ratio openings in silicon oxide having a patterned photoresist mask layer thereover. The etchant of the invention does not degrade the profile of the mask pattern; provides excellent selectivity between the mask layer and the silicon oxide; and provides excellent selectivity between the silicon oxide and a silicon-containing substrate material. The etchant of the invention eliminates the stop etch problem as well.
The etchant of the invention comprises a plasma formed from a mixture of a fluorocarbon and an organic silane that includes hydrogen, silicon and carbon.